Gravitational Microlensing

The galactic microlensing group at Jodrell Bank is one of the most active in Europe
and undertakes both theoretical work as well as analysis of
microlensing datasets.
We have close links both with the main microlensing
survey groups (OGLE and MOA), as well as with follow-up groups who are using
the technique to look for extra-solar planets. Staff here have pioneered
both the theory behind extra-solar planet detection with microlensing, as
well as the application of the technique beyond the Milky Way. Read on for a
brief overview of microlensing as well as a summary of projects being
undertaken here.

What is microlensing?

A microlensing event shown in false colour. The event is
the red flash
above the bright
foreground star. It was observed in the Andromeda Galaxy in 1999 by the
POINT-AGAPE dark matter microlensing survey.
Click on the image for a movie of the event.

Gravitational lensing describes the
deflection and distortion of light by intervening mass distributions. The
size of the effect depends upon the geometry of the observer, lensing mass
and
background source, as well as on the mass, size and shape of the lensing
object.
On large distance and mass scales, foreground clusters of galaxies can
produce
multiple distorted images of background galaxies, sometimes with
spectacular results that are akin to a "hall of mirrors" effect.
Many of these lenses are studied by the Gravitational Lensing
Group here
at Jodrell.
On much smaller scales, such as
within galaxies, gravitational lensing signatures can be caused by the
close passage of stars or planets across the line of sight to more
distant stars. In these cases multiple images are also produced but
are typically separated by around a milli-arcsecond and so
are too close together on the sky to be resolved. Instead, the combined
brightness of the
images is observed to vary with time in a very precise way which is
wavelength
independent. This regime of transient, unresolved gravitational lensing
is known as microlensing.

Microlensing within galaxies is an extremely rare phenomenon.
Even
towards the most crowded stellar fields in the bulge of the Milky Way
there are typically one or two detectable microlensing events ongoing at any given
time for
every million stars observed. This led Einstein largely to dismiss
the possibility of being able to detect microlensing events when he
wrote
about the subject in 1936. Worse still, there are approximately 1000
intrinsically variable stars for every detectable microlensing event.
Fortunately, the
precise transient behaviour of microlensing events allows them to be
reliably distinguished from variable stars, and advances in detector
technology and
computing power allow today's astronomers to routinely monitor and
analyse tens of
millions of
stars each night. To date the efforts of several microlensing surveys of
the Milky Way,
such as
EROS,
MACHO,
MOA and
OGLE
have yielded several thousand microlensing events, almost all towards the
Galactic bulge. Members of the Jodrell Microlensing Group
have been involved in both of the currently ongoing main microlensing surveys,
OGLE and MOA, which are responsible for the large majority of events
detected to date. Einstein would probably have been surprised to learn that,
seventy years on, microlensing is by far the most commonly observed form of
gravitational lensing.

Microlensing samples have been used to measure the density of compact
(Macho) dark matter candidates in the halo of the Milky Way and the
Andromeda Galaxy, to
probe the three-dimensional structure of the Galactic bulge, to identify
black hole candidates, to resolve the photospheres of distant stars, and
to detect extrasolar planets. Astronomers at Jodrell have been and
continue to be at the
forefront of these efforts.

Microlensing at Jodrell

Extra-solar planets

An artists impression of the 5-Earth mass planet discovered recently
through the microlensing technique.

(Mao & Rattenbury)

In 1991 a seminal
paper by Mao & Paczynski explored the
possibility of detecting planets using the microlensing effect. The idea
they put forward was that the presence of a planet orbiting the lensing
star could, under a favourable alignment, induce a
detectable perturbation on the microlensing lightcurve. This theoretical
possibility became an observational reality 13 years later with the
first
detection of a microlensing planetary system.

Recent
discoveries of low-mass planetary companions using microlensing
have been achieved due to real-time alerts issued by the OGLE and MOA
survey teams. The alerts are monitored intensively by follow-up networks
such as PLANET, RoboNET and MicroFUN.
Jodrell astronomers are members
of both the main survey teams as well as the RoboNET robotic follow-up
network. Results from these surveys indicate that planets in the Earth to
Neptune mass range may be much more common than predicted by theoretical
models of planet formation. Microlensing is currently the most sensitive
technique for the discovery of such planets, having recently detected a
planet with a mass around 5
Earth masses.

HST follow-up studies of microlensing events

HST images taken 3.7 years (left) and 8.9 years (right)
after an observed microlensing event. The lens and source are clearly resolved in the later image (components A and B).

(Kozlowski, Mao & Wood)

Whilst the microlensing effect can produce strong changes
in the brightness of the background source, the foreground lensing star need
not be visible at all. Nonetheless some events should, in
principle, involve detectable lensing stars. However, the difficulty
in identifying the lensing star is that its close proximity on the sky to the
source star
during the event prevents it from being directly resolved.
Nonetheless, microlensing surveys have been monitoring the brightness of
millions of stars in the Galactic bulge since the early 1990s and so
differences in proper motion should allow the stars to be resolved after several years.

Astronomers at Jodrell have recently used the Hubble Space Telescope (HST) to directly identify the lensing star
involved in a microlensing event which occurred nine years previously. This is the
first time a microlensing star has been directly identified. The importance of this
identification is that it allows the nature of the lens to be established. Furthermore, it
permits a consistency check on the microlensing hypothesis by back-tracking the observed
lens and source trajectories and checking for consistency with the lensing
geometry inferred
from the original event data.

Galactic structure from microlensing datasets

A simulation of the predicted distribution
of optical microlensing events across the Galactic bulge. This is the
first such simulation
to incorporate a realistic 3D dust model.

(Kerins, Mao & Rattenbury)

Microlensing is sensitive to the line-of-sight geometry of stars as
well
as their transverse kinematics. This make microlensing datasets
potentially
powerful probes of Galactic structure.

At Jodrell we are using microlensing datasets to probe the 3D structure
of the inner Galaxy. Using state-of-the-art synthetic stellar population
models we are constructing detailed theoretical maps of the distribution
of
microlensing events. Our latest maps include 3D models for
interstellar absorption.

Microlensing surveys also provide accurate photometry for large samples of
standard candle stars such as red clump giants. Their magnitude
distribution has been used recently to constrain the
orientation and
physical parameters of the Galactic bar.

At Jodrell we are also investigating the possibility of infrared
microlensing surveys. To date all microlensing surveys have been conducted
at optical wavelengths. Unfortunately our location within the disk plane
means that large areas of the Galactic bulge are obscured at visible
wavelengths by intervening dust. This means that microlensing surveys have
hitherto been confined to regions of the bulge where the dust column density
is relatively small. Theoretical models developed at Jodrell show that
microlensing surveys can trace the underlying Galactic structure much better
at near-infrared wavelengths, such as in the K band (centred at 2.2μm).

Using K-band data obtained from the WFCAM array on UKIRT, currently the World's
largest infrared telescope, we have demonstrated that near-infrared
monitoring provides a powerful step forward for galactic structure studies
with microlensing. The next step forward is a major large-scale near-infrared
variability survey of the Galactic bulge and disk, dubbed VVV, which is to be undertaken
from 2009 on the new VISTA infrared telescope in
Chile. VVV will be able to detect microlensing right across the bulge.
Microlensing samples from VVV will trace the bulge
structure far better than can be achieved with current optical surveys.

Extra-galactic microlensing

Our birds-eye view of the Andromeda Galaxy,
our nearest giant galactic neighbour, makes it a
good choice to look for microlensing events. Its triaxial bulge is the focus for the
Jodrell-led Angstrom Project (NOAO image).

(Kerins)

Microlensing events are also detectable in galaxies other than our own,
however the challenges of detection are more difficult. In general the
source stars to extragalactic microlensing events are unresolved in
the absence of lensing. As a result we see only the peak of the
microlensing event when it is highly magnified above the galactic
background starlight. In order to obtain stable photometry of the
event, image enhancement techniques must be employed to correct for
changing atmospheric conditions. This kind of unresolved microlensing
is sometimes referred to as pixel lensing.

The Angstrom Project
is a Jodrell-led microlensing survey of the bulge
region of the Andromeda Galaxy (M31). This unique international
collaborative project employs a
distributed network of five 2-metre-class telescopes to monitor the M31
bulge
24 hours per day. The intensive monitoring is necessary to see the
relatively short-lived peaks of ongoing microlensing events. Two of the
telescopes in the network are fully robotic and the data from them are
analysed by a fully-automated data reduction pipeline. The Angstrom
Project Alert System analyzes tens of thousands of lightcurves
daily to look for signals of ongoing microlensing candidates or other
transient objects such as classical novae. The Angstrom Project is the
first survey ever to detect microlensing events without any
human intervention from the point of observation to the point at which
a candidate is flagged up. It is also the first extragalactic
microlensing survey to employ real-time data reduction. We aim to use
the spatial and timescale distribution of M31 microlensing events to probe
the 3D
structure of the triaxial bulge in M31. Using real-time data reduction
we also intend to issue alerts of ongoing events. Our
aim is that some of the exciting science being performed by real-time
microlensing discovery within our own Galaxy will also be possible with
extragalactic microlensing systems, possibly including the discovery of
extra-galactic
planetary systems.